A toothed belt used for a camshaft drive for an internal-combustion engine of an automobile is required to have high dimensional stability in order to maintain the appropriate timing. Alternately, a rubber belt used for driving an injection pump or the like, or transmitting power in an industrial machine or the like is required to have strength and elasticity that can withstand a high load and severe bending. In order to satisfy such needs, in some cases, a reinforcing cord including reinforcing fibers has been embedded in the matrix rubber of these rubber products.
In the reinforcing cords, a mixed liquid (RFL liquid) of a resorcinol-formalin condensation product and a latex generally is used for protecting surfaces of the reinforcing fibers. In this case, the RFL liquid is applied to fibers before or after a primary twist. Next, a plurality of primarily twisted fibers are bundled and then finally twisted. Furthermore, in order to improve the bonding property, another coating layer may be formed according to the kind of matrix rubber. Thus, the reinforcing cord is produced.
Various kinds of treatment agents have been studied for improving the performance of such reinforcing cord. For example, for the purposes of improving heat resistance, a cord using an RFL liquid that contains an H-NBR latex has been proposed (JP 63(1988)-270877 A). On the other hand, for the purposes of improving the bonding property with the matrix rubber, a cord whose surface is covered with a coating layer that contains a rubber component and maleimide has been proposed (JP 11(1999)-241275 A).
However, when these rubber products are subjected to bending stress repeatedly at high temperatures, or suffer penetration of water, oil, or the like, a peeling-off may occur between the reinforcing cord and the rubber matrix, or a crack may be generated inside the reinforcing cord.
Generally, in the RFL liquid applied to the primarily twisted reinforcing fibers, the reaction is almost entirely completed by thermal treatment. Therefore, the primarily twisted fibers within the reinforcing cord do not bond to one another but are merely in contact with one another. These fibers are not bonded. For this reason, when the rubber product is bent, gaps are formed among the primarily twisted fibers inside the reinforcing cord. As a result, the strength of the cord has deteriorated in some cases. The deterioration in strength becomes apparent when, in particular, water or oil penetrates into the rubber. Therefore, when the rubber product is used under severe conditions, the durability of the cord should be improved further.